1. Field of the Invention
The present invention relates to code division, multiple access (CDMA) telecommunication systems, and, more particularly, to scheduling downlink data transmission from a base station to users.
2. Description of the Related Art
Mobile cellular or wireless systems of the prior art generally have limited capabilities for transmission of data signals (data transmission) between a base station and remote users in comparison with transmission of voice or voiceband data signals. For some code division, multiple-access (CDMA) communication systems, such as those systems presently operating in accordance with the IS-95 standard, transmission of data traffic to and from users may only have a data rate of approximately 10 kbits/s. This data rate may typically be too low to satisfy delay requirements of many data applications, including connections between a remote user and the Internet or for file transfer to/from the remote user. When a large number of voice users are supported, user connections of the cellular or wireless network may be relatively easy to manage using statistical averaging. However, statistical averaging techniques may no longer be valid when a few users having high data rate traffic are admitted into the network.
Modifying resource management of existing CDMA telecommunication systems may allow a network to admit users having high data rate traffic. Resource management primarily includes schedule time, transmit power, and data rate. One approach to resource management in such CDMA networks schedules the transmission of data traffic from users to the base station (i.e., transmissions in the up-link channel) in a manner to avoid interference between user signals. Increasing the transmission rate while avoiding interference from other users compensates for a reduction in available transmission time.
FIG. 1 shows CDMA telecommunication system cell 100 comprising a base station 101 and voice and data users. Cell 100 operates in accordance with prior art methods of resource management. Base station 101 is in communication with transceivers of voice users xV1 and xV2 and data users xD1 through xD3. Base station 101 has maximum transmission power Pmax available for transmission from the base station 101 to users in a down-link channel. To maintain a rate and minimum bit error rate (BER), the base station 101 communicates with data users xD1 through xD3 during a time period with a certain data rate and transmit power level.
For the up-link channel, methods of the prior art may use common CDMA techniques, such as overlapping techniques (simultaneous transmission of spread signals within the same frequency band during the same time) that allow users to transmit signals to the base station 101. For both up-link and down-link channels, a BS 101 of the prior art may employ overlapping techniques for transmission of data as shown in FIG. 2. During time span xcex94t, the base station 101 apportions fractions xcfx86D1 through xcfx86D3 of the total transmit power PD to user transceivers xD1 through xD3. Consequently, each user receives interference noise from other users with transmissions occupying the same frequency band during the time span xcex94t. While providing continuous transmission of data, the overall throughput of a cell may be reduced because of the lower signal to interference noise level.
Also, methods may schedule each voice and data signal transmission between users and a base station to reduce interference noise, and these methods may employ non-overlapping scheduling techniques (i.e., spread user signals within a frequency band are transmitted either one-at-a-time, or in small, simultaneous groups). This method is used by a base station to coordinate many signals transmitted between the base station and users, which signals may experience varying forms of delay during transmission. Methods of scheduling signal transmission generally apply one or more criteria to determine which particular user signal should be transmitted. However, voice/voiceband data signals are delay non-tolerant, as opposed to data signals which are generally delay-tolerant, and as such voice signals may be adversely affected by delay caused by scheduling methods. Consequently, scheduling methods may divide user signal transmission into one or more classes depending on whether the spread signals are delay tolerant or delay intolerant.
A method of intra-cell scheduling of the prior art is described in J. M. Holtzman and S. Ramakrishna, A Scheme for Throughput Maximization in a Dual-Class CDMA System, I.E.E.E. Journal on Selected Areas of Communications, 40(2): 830-841, 1998. For this method, the base station schedules user transmissions in the up-link since the overlapping, asynchronous transmissions contribute significant noise to one another since the spreading codes are not aligned. Such scheduling may be employed to improve gain (SNR) in the up-link by reducing high interference noise power from non-zero partial cross-correlation of the transmitted signals. Another prior art intra-cell scheduling method may allocate transmission slots in the down-link for signal transmission from the base station to users using signal-to-noise (SNR),measurements and/or fading predictions. A user measures the SNR value of the signal received at the user transceiver and transmits the SNR value back to the base station. The base station then allocates the transmission slot based on maximizing the SNR while providing the minimum transmission delay for all users. Such scheduling methods base such allocation on various throughput criteria of the cell, and these systems may be characterized by low user speed and are generally designed for data-only (delay-tolerant user) transmission.
The present invention relates to intra-cell scheduling of transmission by a base station of one or more spread data signals to corresponding users in a cell. Each of the one or more users are allocated a recurrent time interval during which transmission of each spread data signal from the base station to the corresponding user is enabled. Before allocating the recurrent time intervals, a linear programming problem is solved that characterizes the cell, thereby to determine the recurrent time interval for each of said users. After allocating the recurrent time intervals, each of said one or more spread data signals are transmitted from the base station to the corresponding user during an allocated time interval that does not overlap the time interval allocated to any other of said one or more users, such that a transmission is made only one at a time of said one or more users.
For some embodiments of the present invention, the base station transmits each spread data signal with a transmit power that is near a relative maximum transmit power of the base station. Some embodiments of the present invention may determine the time intervals and transmit powers allocated to spread data signals based on either minimum average rates, an optimization criterion such as relative maximum throughput, or signal-to-interference noise measurements by users. According to other embodiments of the present invention, users having delay-intolerant spread signals may be allocated corresponding time intervals that may overlap time intervals of transmitted spread data signals. Other embodiments of the present invention may allocate a common time interval to a set of users during which the spread signals are transmitted in time intervals that may overlap one-another, and the spread data signals of another set of users are transmitted in corresponding time intervals that are disjoint.